How clever are you?

Take this simple Observer quiz to find out your overall intelligence level

Although a full IQ test takes an hour or more and costs hundreds of pounds, performance on this one simple test is highly predictive of your overall intelligence.

Look at the four cards above. We know for a fact that each card has a letter on one side and a number on the other. Truthful Terry says: ‘Every card that has a D on one side has a 3 on the other.’ What is the fewest number of cards you need to turn over to find out whether Truthful Terry is actually telling the truth? And which ones? The answer is shown at the bottom of the page.

If you got it right, congratulations! Either you’ve seen this before or you’re a candidate for Mensa.

Seriously, I’ve given this test to hundreds of people up and down the country, and very few get it right (the exception was an audience of Google software engineers, not a single one of whom messed up). The analytical and deductive skills you’ve shown are one of the most important components of intelligence.

If you slipped up, don’t worry. The reason is a phenomenon known as confirmation bias. Most people, being charitable sorts, want to turn over the 3, find a D on the back and say: “Well done, Terry!” Actually it doesn’t matter whether the letter on the back of the 3 is D, Z or something else (look back to what he actually said).

Confirmation bias – the tendency to try to confirm pre-existing beliefs – is one reason many people believe complete nonsense (eg homeopathy).

Ironically the only way to check the truth of Terry’s statement is to try to disconfirm it – ie to try to find evidence that he is wrong (a D on the other side of the 7). Here’s the intel: tinyurl.com/pmkl296.

Answer

Two cards, D and 7

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